1. Field of the Invention
The present invention relates to the field of semiconductor substrate processing equipment. More particularly, the present invention relates to an apparatus and method for directing a purge gas at an angle to the radial direction of a substrate to prevent deposition in the exclusionary zone near the edge, on the edge, and on the back side of the substrate without interfering with the deposition of the desired film on the substrate.
2. Background of the Related Art
In the fabrication of integrated circuits, equipment has been developed to automate substrate processing by performing several sequences of processing steps without removing the substrate from a vacuum environment, thereby reducing transfer times and contamination of substrates. Such a system has been disclosed for example by Maydan et al, U.S. Pat. No. 4,951,601, in which a plurality of processing chambers are connected to a transfer chamber. A robot in a central transfer chamber passes substrates through slit valves in the various connected processing chambers and retrieves them after processing in the chambers is complete.
The processing steps carried out in the vacuum chambers typically require the deposition or etching of multiple metal, dielectric and semiconductor film layers on the surface of a substrate. Examples of such processes include chemical vapor deposition (CVD), physical vapor deposition (PVD), and etching processes. Although the present invention pertains primarily to CVD processes, it may have application to other processes as well.
Vacuum chambers are employed to deposit thin films on semiconductor substrates. Typically, a precursor gas is charged to a vacuum chamber through a gas manifold plate situated above the substrate, which substrate is heated to process temperatures, generally in the range of about 250 to about 650.degree. C. The precursor gas reacts on the heated substrate surface to deposit a thin layer thereon.
To increase manufacturing efficiency and device capabilities, the size of devices formed on a substrate has decreased, and the number of devices formed on a substrate has increased in recent years. Also, it is increasingly important that the thin films be of uniform thickness across the substrate so that all of the devices on the substrate are uniform. Further still, it is increasingly important that the generation of particles in processing chambers be avoided to reduce contamination of substrates that will reduce the yield of good devices.
In a typical process chamber, a support member on which a substrate is mounted during processing is movable vertically in the chamber. The substrate is brought into the chamber from an external robot blade. A plurality of support fingers are also vertically movable by an elevator and extend through the support member to facilitate transfer of the substrate from the robot blade to the support member. In most CVD processes, the substrate and the support member on which it is supported are typically heated.
Such process chambers may be used to deposit metals, such as tungsten, from WF.sub.6 precursor gas onto the substrate as well as other metals and dielectrics. WF.sub.6 is a highly volatile gas, and problems have arisen because tungsten deposits not only on the top side of the substrate, but also on the edge surfaces and back side of the substrate. These edge and back side surfaces are typically rougher than the highly polished top surface and are not coated with an adhesive layer such as sputtered titanium nitride and, thus, the deposited materials tend to flake off the edge and bottom surfaces of the substrate, thereby contaminating the chamber. Also, material deposited on these surfaces may cause the substrate to adhere to the support member and may compromise the integrity of the devices formed near the edge of the substrate. Additionally, some processes use a barrier metal film that is, for example, formed of titanium and titanium nitride that covers the entire substrate surface wherein the titanium layer is partially exposed. Often in these processes, a layer of tungsten is deposited on the barrier layer. However, as tungsten is not adhesive to titanium, the tungsten deposited thereon tends to exfoliate therefrom, thereby creating particles.
Thus, shadow rings and purge gas have come into use. Shadow rings cover the periphery of the substrate during deposition to mask this area of the substrate, thereby preventing the deposition gases from reaching the edge and back side surfaces of the substrate. However, due to the volatility of WF.sub.6, for example, shadow rings alone do not prevent edge and back side deposition on the substrate. The use of a purge gas directed behind or at the edge of the substrate behind the shadow ring has therefore been tried. The purge gas exerts a positive pressure that reduces the likelihood that processing gas will reach these edge and back side surfaces. In systems using a purge gas, the support member has a plurality of spaced purge gas orifices extending therethrough that deliver the purge gas to an annular gas groove in the upper surface of the support member. The annular gas groove surrounds the substrate and delivers the gas to the peripheral edge of the substrate. Typically, this gas is delivered to the substrate from below the edge so that it flows around the edge of the substrate and effectively flows over the upper surface of the substrate in a direction perpendicular to the edge of the substrate. One example of a shadow ring and gas passage is shown in U.S. Pat. No. 5,516,367 which issued on May 14, 1996, which is hereby incorporated by reference.
As the desire for greater throughput and efficiency has increased, the standards governing the thickness and uniformity of the deposited film at the substrate edge have continually become more stringent. Ideally, the deposited film has a uniform thickness across the full area of the substrate with the edges dropping off rapidly so that the zone of exclusion has little or no deposition thereon. Further, there is ideally no deposition on the beveled edges of the substrate. Industry practice has moved toward this ideal goal so that the current industry standards demand no film deposition on the beveled edge of the substrate and a film thickness at a point 3 mm from the edge of the substrate that is 90 percent or more of the film thickness at the center of the substrate with a thickness uniformity of .+-.5 percent, excluding the area within 5 mm from the substrate edge.
One important element in meeting the requirements involves optimization of the purge gas delivery and its flow rate. If the flow of purge gas is too great, the purge gas may prevent or interfere with the deposition of the process gas on the substrate where deposition is desired. As the flow rate rises, the purge gas flows further inward toward the center of the substrate causing greater interference with the process gas flow and its even distribution. Therefore, the interference may cause film uniformity problems near the edge of the substrate as well as further toward the center of the substrate. Accordingly, a high flow rate of purge gas may prevent the process from meeting the uniformity requirements of industry standards. However, a relatively high flow rate may be needed to increase the positive pressure produced by the purge gas in order to meet the industry requirements for edge exclusion and for prevention of deposition on the edge and back side surfaces of the substrate.
Therefore, there is a need to provide a system which prevents back side deposition and meets the edge exclusion demands of the industry while not affecting deposition uniformity across the surface of the substrate.